The invention relates to a method and apparatus for establishing a connection link set up in shortwave radio networks having several stations with one transmitter and/or one receiver, by means of a call signal sent out from a transmitter and including a synchronization signal and an address signal.
Shortwave connections primarily use the spread of skywaves which are reflected at the ionosphere in order to realize the transmission of news over great distances. In spite of the insufficiencies of the transmission channel for a skywave connection, such as noise-like channel interferences, time-variant, dispersive channel behavior and the presence of selective sources of interference, this means of transmission has recently enjoyed a considerable increase in importance thanks to new microprocessor techniques and, by comparison with satellites, low cost.
Special problems occur during establishing of the connection because there is always a greater or smaller frequency difference (offset) between transmitter and receiver frequencies and because no time synchronization is given before the connection between transmitter and receiver is taken up.
Transmissions usually result in the economic use of the frequency band by means of single side band technology, in which at the transmitter end, a frequency translation of the signal is undertaken out of the acoustic frequency (300 Hz to 3.4 KHz) into a chosen high frequency band and the reverse operation is carried out by the high frequency receiver. The received signal is passed on in the low frequency region to demodulator and decoder circuits. The high frequency receivers dispose of automatic gain control, in which the total power or voltage within the chosen receiving channel band width constitutes the steering signal. In the process, depending on the spectral covering of desired and interfering signals, noise and desired levels varying between broad limits appear at the output.
In a selective call network various stations are to be activated either individually or with a collective word. The selective call transmitters and receivers of the individual stations are accommodated in their modulator or demodulator block. The call signals are composed of a group of suitable amplitude-time-functions, which can be recognized in the channel noise and distinguished from one another by the individual receivers. Even in transmissions of low quality, on the one hand, unwanted stations should never be activated and, on the other hand, the wanted stations should always be activated.
Pilot tone transmissions usually are not capable of fulfilling these requirements because the probability of faulty synchronization increases with the presence of certain interferences.
It is known how to use a relatively costly equipement system in addition to the transmitter-receiver parts of the stations, with which it may be determined which channel is free and undisturbed by neighboring transmitters, and what are the momentary spreading conditions over the ionosphere (see for example, Frequency Management System, Defense Electronics, May 1980, pp. 21, 22). However the fully automatic establishing of a shortwave connection is not possible with this system. In the case of intended interferers (ECM), a prior analysis is only of little use, as in every case the channel in use is immediately jammed.
The foregoing illustrates limitations known to exist in present devices. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.